1. Field of the Invention
The invention is related to a method for metering an ammonia-releasing reducing agent into the exhaust gas stream of an internal combustion engine installed in a motor vehicle and operated with excess air, wherein a control unit meters the quantity of reducing agent as a function of a stored model and during the operation of the internal combustion engine varies the metered quantity during certain operating phases and compares the change in the measured value of at least one NOx sensor installed downstream of the SCR catalyst with an expected value.
2. Description of the Related Art
In addition to solid particles, nitrogen oxides belong to the legally restricted exhaust gas components that form during combustion processes. The permitted levels of these components is continually being lowered. Various methods are in use today to minimize these exhaust gas components in the internal combustion engines operated in motor vehicles. Lowering the levels of nitrogen oxides is usually done with the help of catalysts. To raise the selectivity and the NOx conversion rates, a reducing agent must also be present in the oxygen-rich exhaust gas.
These approaches have become known under the collective term “SCR method”, where SCR stands for “selective catalytic reduction”. They have been used for many years in the power plant industry and recently also in internal combustion engines. A detailed description of these methods can be found in DE 34 28 232 A1. V2O5-containing mixed oxides such as those in the form of V2O5/WO3/TiO2 can be used as SCR catalysts. The amounts of V2O5 present are typically in the range of 0.2-3%. The use of iron-containing and/or copper-containing zeolites is also conceivable.
Ammonia or compounds, which split off ammonia such as urea or ammonium formate in solid form or in the form of a solution, are used as reducing agents in practical applications.
Urea decomposes at high temperatures into isocyanic acid and ammonia:(NH2)2CONH3+HNCO  (1)
The isocyanic acid is hydrolyzed by water in the exhaust gas to NH3 and CO2:HNCO+H2ONH3+CO2  (2)
Upon complete hydrolysis of one mole of urea, therefore, two moles of ammonia and one mole of carbon dioxide are formed:(NH2)2CO+H2O2NH3+CO2  (3)
As a result, the hydrolysis of urea makes available the same reliable reducing agent used in the power plant industry, namely, ammonia.
One mole of ammonia is required to convert one mole of nitrogen monoxide:4NO+4NH3+O24N2+6H2O  (4)
The ratio of NH3 to NOx is called the feed ratio and respectively.α=NH3/NOx  (5)
In the case of an ideal catalyst, this means that, at a feed ratio of one, all of the nitrogen oxides are reduced; that is, a NOx conversion of 100% is achieved, because the following is true for NOx conversion XNOx:XNOx=(cNOx,0−cNOx)/cNOx,0 
where                CNOx,0 raw NOx emissions, ppm;        CNOx: NOx emissions after the catalyst, ppm.        
If, to form NO2, a platinum-containing NOx oxidation catalyst is installed upstream of the SCR catalyst:2NO+O22NO2  (6)
then the SCR reaction can be greatly accelerated, and the low-temperature activity can be significantly increased.NO+2NH3+NO22N2+3H2O  (7)
Nevertheless, in the presence of NO2, it must also be expected that the emissions of nitrous oxide will also increase according to the following reaction:2NH3+2NO2+½O22N2O+3H2O  (8)
In the case of internal combustion engines operating in motor vehicles it is difficult to use the SCR method to lower the nitrogen oxides, because the operating conditions are always changing. For example, the exhaust gas temperatures, the quantities of exhaust gas, and the raw NOx emissions are subject to frequent fluctuations. This makes it difficult to add the proper quantities of the reducing agent. On the one hand, the goal is to achieve the highest possible conversion of nitrogen oxides, but at the same time care must be taken not to allow the emission of nitrous oxide, isocyanic acid, or unconsumed ammonia.
To meter the reducing agent for the SCR method in motor vehicles, there are currently two different ways in which the correct metered quantity of reducing agent is determined.
The first is a pure open-loop control method without any feedback for determining the actual emissions downstream of the catalyst system. The metered quantity is determined in this case with the help of models based on data which are acquired and/or stored in the memory of an electronic engine control device of the internal combustion engine in the form of tables, curves, characteristic diagrams, or functions and possibly with the help of sensors for determining the catalyst temperature and the quantities of NOx and exhaust gas. The raw emissions of the engine are calculated, for example, from the injected quantity, the engine rpm's, the injection pressure, and fuel/air ratio, etc. The possible NOx conversions and the metered quantities of reducing agent required to achieve them depend in turn on the catalyst temperature, on the raw NOx emissions, on the quantity of exhaust gas, etc. The actual emissions downstream of the system are not detected and thus have no effect on the metered quantity (DE 43 15 278 A1, DE 195 36 571 A1, DE 199 06 344 A1, EP 898 061 A1).
A disadvantage of this method is that, because of the absence of feedback concerning the actual emissions, it is almost impossible to compensate for errors, defects, or environmental influences.
The second possibility is a standard closed-loop control circuit with NOx, sensors downstream of the system. For this purpose, the actual values being supplied currently by the sensors are compared with the nominal values, and the metered quantity is adjusted continuously.
Nevertheless, the problem of permanent closed-loop control consists in the inertia of the system and of the sensors and simultaneously in the highly dynamic way in which an internal combustion engine operates in a motor vehicle. For example, during an acceleration process or an increase in the load on an exhaust gas-turbocharged internal combustion engine, the NOx emissions can rise by a factor of 10 within one second. In the case of naturally-aspirated engines, the rise occurs even faster because of the lack of inertia of the exhaust gas turbocharger. The same is also true when loads are shed or on the transition to operation in push mode.
The sensors used to determine the emissions are not able to detect these highly dynamic processes. One of the reasons for this is the inertia of the sensors. The typical t90 time, that is, the time at which 90% of the end value is reached, of these sensors is found in the range of 300-500 ms. Another reason is the necessity to position the sensors behind the catalyst system. Thus the gas transit time from the discharge point from the cylinder head to the discharge point from the catalyst system alone is in the range of 200-2,000 ms, depending on the volume flow rate of the exhaust gas and the volume of the exhaust gas system.
One possibility of partially solving this problem is to add up or to integrate the nominal and actual emissions over a relatively long period of time and to adjust the metered quantity on the basis of the difference between nominal and actual (DE 101 00 420 A1).
The NOx sensors required for closed-loop control are described in JP 63038154 A, JP 10062374 A, and JP 9288084A. Common to all these sensors is their high cross-sensitivity to reducing exhaust gas components. This is especially problematic when sensors of this type are used in SCR systems, because large quantities of the strong reducing agent ammonia can be present in the exhaust gas. Because ammonia delivers a signal as strong as that of NOx, it is not possible to distinguish between NOx and NH3; that is, strong sensor signals can correspond to high NOx concentrations and/or to high NH3 concentrations. If unconsumed ammonia is emerging downstream of the SCR catalyst, it is for this reason no longer possible to arrive at a specific NOx concentration downstream of the SCR system by closed-loop control.